3. Source Assessment
3. SOURCE ASSESSMENT
Diazinon has been identified as a cause of aquatic toxicity observed in San Francisco Bay Area urban creek water. This assessment summarizes what is known regarding the sources and conveyances of diazinon. It describes the magnitude of diazinon use in the Bay Area, available formulations, common application sites, and target pests, and discusses how this information pertains to pesticides more generally.
SOURCES OF PESTICIDES IN URBAN CREEKS
Figure3.1 illustrates the conceivable pathways through which a generic pesticide applied in an urban area could reach surface water. In the specific case of diazinon discharges to Bay Area urban creeks, the predominant pathways are storm water runoff, dry weather discharges from storm drains, and possibly direct discharges (e.g.,dumping) (CDPR 2001b). This conclusion follows from the elimination of the other possible pathways suggested in Figure3.1, as discussed below.
FIGURE 3.1
Conceivable Pathways for a Generic Pesticide
to Reach Surface Water
In the Bay Area, wastewater treatment plants and industrial and commercial facilities do not typically discharge into urban creeks. Their discharges flow directly to San Francisco Bay or the Pacific Ocean. Shipping and boating do not typically involve diazinon use and do not occur in Bay Area urban creeks; therefore, they are not known sources. Watersheds upstream from Bay Area urban areas are primarily open space; consequently, upstream flows are not a major source of diazinon in Bay Area urban creeks. Air deposition could contribute diazinon to upstream flows, but air deposition is primarily a conveyance mechanism for diazinon from other sources (see Section4, Linkage Analysis). Sediment is another type of conveyance. It carries diazinon from place to place when a diazinon-laden particle reaches a creek or forms within a creek. Groundwater is also not believed to be a significant source of diazinon in urban creeks because diazinon adheres strongly to particles and is seldom found beyond the top 0.5 inches of affected soil (ETN 1996). Diazinon has been detected in less than 2% of shallow groundwater samples from urban areas, with the highest level reported being 10 nanograms per liter (ng/l, parts per trillion) (U.S.EPA 2000f).
While direct discharges to surface water could occur, most diazinon discharges flow to storm drain systems. The relative size of the urban areas draining directly to creeks via overland flow is very small compared to the relative size of urban areas draining to storm drains. Diazinon discharges resulting from random illicit activity or accidental spills, therefore, are far more likely to flow into a storm drain system than directly into a creek. Regardless of this distinction, however, the pest management activities that result in direct diazinon discharges to urban creeks and discharges to storm drain systems are essentially the same. Therefore, this report does not address them separately.
For the reasons stated above, storm drain systems are believed to be the sources of essentially all the diazinon in urban creeks. Storm water runoff and dry weather discharges both flow through storm drains systems. For a particular creek, the storm drain systems that flow into that creek are the sources of diazinon to that creek.
Storm drain systems are regulated as point sources, and in large urban areas, including most of the Bay Area, storm drain systems are subject to National Pollutant Discharge Elimination System (NPDES) permits. Of the Bay Area counties in which urban creeks are considered impaired, Alameda County, Contra Costa County, San Mateo County, and Santa Clara County have countywide NPDES permits for their storm drain systems. In Solano County, the Cities of Vallejo, Fairfield, and Suisun have NPDES permits for their storm drain systems.
REPORTED AND UNREPORTED DIAZINON APPLICATIONS
Diazinon does not naturally occur in the environment; it is manufactured. Diazinon was originally registered for use with the U.S.Environmental Protection Agency in 1956. Syngenta manufactures diazinon and is currently the lead registrant. Other manufacturers and formulators (companies that formulate commercial products with the diazinon manufactured by others) include Mahketshim-Agan, Drexel, Prentis, Gowan, and Aventis (U.S.EPA 2000a). As shown in Figure3.2, these companies sell diazinon products to distributors and retailers. Retailers then sell them to the agricultural users, structural pest control operators, professional landscape maintenance gardeners, and private citizens who apply them. In the Bay Area, the diazinon in runoff flowing to urban creeks through storm drain systems results from these diazinon applications.
Diazinon is the active ingredient in many pesticide product formulations. Most of these formulations also contain so-called “inert” substances at various concentrations. This
FIGURE 3.2
Parties Responsible for Pesticides in Urban Creeks
report uses the term “diazinon” to refer to the active ingredient only, not to the entire product. The many inert ingredients are not considered when quantities of diazinon are discussed below.
California requires all agricultural pesticide applications to be reported to local Agricultural Commissioners. The California Department of Pesticide Regulation, in turn, compiles these data. “Agriculture” is defined to include applications on parklands, golf courses, rights of way, rangelands, pastures, and cemeteries (i.e.,anything but residential, industrial, and institutional sites). Commercial pest control operators apply diazinon primarily for structural pest control and landscape maintenance, and they must report their pesticide applications. In contrast, private citizens are not required to report applications of products sold over-the-counter at private homes and gardens.
The City of Palo Alto has estimated that, in urban areas, unreported diazinon applications account for up to 60% of all diazinon applications, and reported diazinon applications may represent as little as 40% (Palo Alto 1996). On the basis of estimated sales in Castro Valley and reported applications there, Alameda County has estimated that reported and unreported applications each account for about 50% of all diazinon applications (Alameda County 1997).
As noted in Table3.1, about 85,000 pounds of diazinon were reportedly applied in the nine Bay Area counties each year from 1995 to 2000 (CDPR 2001a; CDPR 2000a; CDPR 2000b; CDPR 1999a; CDPR 1999b; CDPR 1996). When estimating diazinon applications within the San Francisco Bay Regional Water Quality Control Board’s (Regional Board’s) jurisdiction, the difference between the county boundaries used for pesticide reporting and the Regional Board boundaries (which are based on watershed drainage areas) introduces some uncertainty. Some of the nine Bay Area counties straddle Regional Board boundaries, so a relatively small portion of this reported diazinon was applied outside the San Francisco Bay Regional Water Quality Control Board’s jurisdiction. However, areas outside the Regional Board’s jurisdiction tend to be more rural, and areas within the Regional Board’s jurisdiction tend to be more urban. Landscape maintenance and structural pest control are more closely associated with urban areas than most agricultural activities. Therefore, when using county data to estimate reported diazinon applications within the Regional Board’s jurisdiction, the pesticide use reported for structural pest control and landscape maintenance may be only slightly overstated, and the pesticide use reported for agriculture may be substantially overstated.
TABLE 3.1
Reported Diazinon Applications in the Bay Area, 1995-2000
Reported Applications (pounds)Purpose / 1995 / 1996 / 1997 / 1998 / 1999 / 2000
Structural Pest Control / 49,119 / 50,032 / 45,700 / 49,541 / 50,552 / 34,071
Agriculture / 28,113 / 25,214 / 26,061 / 22,034 / 23,271 / 13,472
Landscape Maintenance / 18,500 / 14,468 / 15,961 / 18,274 / 11,382 / 13,740
Other* / 128 / 80 / 777 / 555 / 115 / 1,187
Total / 95,859 / 89,795 / 88,499 / 90,403 / 85,321 / 62,469
* Other uses of diazinon included public health pest control, research commodities, rights of way, and uncultivated areas.
Source: CDPR 2001a; CDPR 2000a; CDPR 2000b; CDPR 1999a; CDPR 1999b; CDPR 1996.
As illustrated in Figure3.3, diazinon applications vary from year to year. These variations may reflect differences in weather, specific pest problems, or recent general trends. Table3.2 shows average reported diazinon applications by county for the period from 1995 through 2000. During this period, more diazinon applications were reported in Santa Clara County than in any other Bay Area county. Contra Costa County ranked second. About 54% of the total diazinon reportedly applied was associated with structural pest control, about 27% was associated with agriculture, about 18% was associated with landscape maintenance, and about 1% was for other types of applications (CDPR 2001a; CDPR 2000a; CDPR 2000b; CDPR 1999a; CDPR 1999b; CDPR 1996).
Given that from 1995 to 2000 an average of 85,391 pounds of diazinon applications were reported each year for the nine Bay Area counties, and assuming that reported and
FIGURE 3.3
Reported Diazinon Applications in the Bay Area, 1995-2000
TABLE 3.2
Average Reported Diazinon Applications by Bay Area County, 1995-2000
Average Reported Applications (pounds)County / Structural Pest Control /
Agriculture / Landscape Maintenance /
Other*
Alameda County / 7,077 / 18 / 1,840 / 3
Contra Costa County / 10,359 / 5,185 / 3,660 / 14
Marin County / 2,485 / 10 / 579 / 0
Napa County / 383 / 90 / 28 / 1
San Francisco County / 597 / 0 / 19 / 0
San Mateo County / 6,053 / 1,028 / 1,187 / 359
Santa Clara County / 15,596 / 7,531 / 6,543 / 65
Solano County / 2,237 / 6,022 / 107 / 30
Sonoma County / 1,714 / 3,142 / 1,425 / 1
Subtotal / 46,502 / 23,028 / 15,387 / 473
Percent of Total / 54% / 27% / 18% / 1%
* Other uses of diazinon included public health pest control, research commodities, rights of way, and uncultivated areas.
Source: CDPR 2001a; CDPR 2000a; CDPR 2000b; CDPR 1999a; CDPR 1999b; CDPR 1996.
unreported applications were each about 50% of the total (Alameda County 1997), then about 85,400 pounds of diazinon applications were probably not reported. The total amount of diazinon applied in the Bay Area, therefore, may have been about 170,800 pounds or about 85 tons per year. On the basis of the data in Table3.2, these 85 tons were probably distributed roughly as illustrated in Figure3.4.
FIGURE 3.4
Distribution of Diazinon Applications in the Bay Area
Alameda County has estimated the annual amount of diazinon applied outdoors to be about 0.02 pounds per person (Alameda County 1997). The population of the Bay Area is about 6,948,000 (a relatively small portion of which reside outside the jurisdiction of the Regional Board) (ABAG 2001). Therefore, Bay Area residents could apply about 140,000 pounds or about 70tons of diazinon outdoors each year. This estimate agrees reasonably well with the above total indoor and outdoor estimate of 85 tons, particularly if most diazinon is assumed to be applied outdoors.
DISTRIBUTION OF DIAZINON WITHIN THE WATERSHED
The distribution of diazinon in urban creeks provides clues about how it is applied in urban areas and the paths it takes to reach surface water. To better understand the distribution of diazinon in urban creeks, the Alameda Countywide Clean Water Program investigated the Castro Valley Creek watershed, which is believed to be typical of many Bay Area urban watersheds. Land uses in that watershed are about 50% low-density residential development, 35% open space, and 15% commercial development (including multifamily residential areas). On the basis of numerous concentration measurements and corresponding flow data, Alameda County estimated the total amount of diazinon discharged to Castro Valley Creek to be about 1.3pounds during the 1995-1996 rainy season. As Figure3.5 illustrates, this load represents a very small fraction (about 0.25%) of the diazinon Alameda County estimated was applied outdoors in the watershed (ACCWP and Alameda County 1997). Assuming that about 0.25% of the 85tons of
FIGURE 3.5
Relationship Between Diazinon Applications
and Diazinon Loads in Urban Creeks
diazinon applied throughout the entire Bay Area finds its way to surface water, the annual diazinon load to all Bay Area creeks is roughly 400pounds.
Analysis of storm water samples collected from the Castro Valley Creek watershed indicated that diazinon applied on surfaces during dry weather appeared to accumulate before washing into the creek during storms. The mass of diazinon discharged to the creek increased with increased flow, although diazinon concentrations decreased, presumably through dilution. Diazinon concentrations were higher in residential and commercial areas compared to those with more open space. Higher diazinon levels were not clearly associated with any particular neighborhoods, however, and diazinon samples from adjacent gutters draining separate residences sometimes exhibited very different concentrations. Alameda County concluded that diazinon comes from multiple, sporadic sources. Individual sources may be very localized, and downstream diazinon levels apparently reflect an average of upstream pulses. At any one time, about 2 to 4% of the properties in residential areas could be contributing diazinon to urban runoff. Some consistent diazinon discharges may also exist in the Castro Valley Creek watershed because some relatively high diazinon concentrations occurred at certain locations during more than one sampling event (ACCWP and Alameda County 1997).
The Alameda Countywide Clean Water Program also studied the San Leandro Creek watershed and came to similar conclusions. Street gutter samples collected from residential areas during a storm exhibited low diazinon concentrations in many areas and high levels in a few areas. Creek samples were more uniform and reflected the average of many different storm water discharges (ACCWP 1999b). The data suggest that diazinon applications at discrete, variable, and independent locations are responsible for the diazinon observed in surface water.
Although most of the diazinon applied in the Bay Area adheres to organic surfaces, degrades in the environment, and is not found in surface water, the relatively small fraction that does reach surface water is responsible for the aquatic toxicity observed in urban creeks. This estimated diazinon load (about 0.25% of the amount applied outdoors) is consistent with runoff that has been observed from routine applications in other areas (Capel et al. 2001). It does not necessarily suggest runoff from isolated and sporadic illicit or accidental activities, although these could also occasionally contribute to the overall load.
The Alameda Countywide Clean Water Program conducted tests to determine if applying diazinon outdoors in accordance with its label instructions could account for observed surface water concentrations. A liquid diazinon concentrate was diluted and applied at a home in accordance with label instructions (except that the amount of diazinon applied was considerably less than the recommended application rate for ants). During subsequent rainfall, runoff concentrations reached as high as 1,200,000ng/l several days after the application. The water quality criterion developed by the California Department of Fish and Game for chronic exposure is 50ng/l, and the criterion for acute exposure is 80ng/l. The highest runoff concentrations occurred when rain closely followed the application, and high diazinon levels persisted for up to seven weeks. The study concluded that applying diazinon in accordance with label instructions could not be ruled out as a source of diazinon in storm water (ACCWP and Alameda County 1997).
FORMULATIONS, APPLICATION SITES, AND TARGET PESTS
Formulations
The roughly 400pounds of diazinon discharged to Bay Area surface water each year is from various products and formulations. Table3.3 provides examples of several existing diazinon formulations. Many other pesticides are formulated similarly. Of the formulations listed in Table3.3, impregnated materials and pressurized liquids, sprays, and foggers are intended primarily for indoor use (although aerosol products may be applied outdoors). The other types of formulations are all applied outdoors.
The relative effect of formulation on water quality depends on (1)how much product is applied and (2)how much of the pesticide in the formulation typically runs off site. Little information is available regarding how diazinon runoff varies with formulation. On the basis of the limited available data, however, wettable powders appear to offer the greatest
potential for concern. Wettable powders are widely applied to impervious surfaces by pest control operators, and when exposed to water (e.g.,rain), they are easily resuspended. Emulsifiable concentrates, granules, and flakes are also common formulations. Studies suggest that emulsifiable concentrates may be more prone to run off than granules and flakes, but not more prone to runoff than wettable powders (CDPR 2001b).
TABLE 3.3
Examples of Diazinon Product Formulations
Formulation / Number of Products* /Product Examples
Granules and Flakes / 80 / Turf products
Emulsifiable Concentrates / 45 / Insect spray concentrates
Impregnated Materials / 35 / Pet flea collars
Pressurized Liquids, Sprays, and Foggers / 21 / Ant and roach sprays and “bombs”
Aqueous (Liquid) Concentrates / 18 / Concentrates for mixing insect sprays
Solutions and Liquids (Ready to Use) / 18 / Ant, roach, and spider sprays for home use
Wettable Powders / 12 / Professional applicator products
Dusts and Powders / 8 / Insecticide dusts
Microencapsulated Materials / 6 / “Controlled release” liquid sprays (concentrates and ready-to-use)
* The number of products includes those registered in California as of September 2000. It does not indicate the relative amount of diazinon applied with each formulation.
Source: CDPR 2001b.
Application Sites
Most over-the-counter diazinon products are applied outdoors. Indoor diazinon applications may result in wastewater discharges, but because Bay Area wastewater treatment plants do not discharge to urban creeks, indoor applications do not result in discharges to urban creeks. A limited survey of retail outlets in Alameda County determined that about 70% of the pesticide products sold there were concentrates, about 30% were granules, less than 1% were dusts, and less than 1% were diluted (i.e.,ready-to-use) products. These products are commonly applied outdoors. Alameda County also interviewed three structural pest control operators. About 90% of their work was for residential properties, and their diazinon applications were exclusively outdoors (Alameda County 1997).
Because diazinon levels in Castro Valley Creek had been studied, and because Castro Valley’s mostly low-density residential development is representative of much of Alameda County, Alameda County conducted a telephone survey of Castro Valley residents to learn more about their pest management practices. The results indicate that about 51% of Castro Valley residents apply some type of pesticide outdoors. Of these, about 35% apply the pesticide themselves, and about 14% hire a professional. As shown in Figure3.6, the pesticides are applied at building foundations, in gardens, along patios and walkways, on trees and shrubs, and on lawns.